There are two ways I can think of: ventilation or a dehumidifier.
Ventilation is only if the outside of your tank is less humid than the inside. You can accomplish this through properly placed fans and/or ventilation holes.
If the outside of your tank is humid as well, the dehumidifier is pretty much your only option.
But, as was mentioned earlier, you need a proper device to detect humidity.
I don't have a water bowl in the enclosure. I am using the zoo med double round temp and humidity gauge, and the humidity is about 40 today, it has dropped down, I left the ceiling fan on! He seems to be doing well, eating, hunting, and running around.
your readings could be off by 20 points unfortunately. You can pick up an electronic probe hygrometer for less than 10 bucks at the store. Analog (dial) thermometers and hygrometers have a hard time getting accurate readings, due to the metal coils that make them work. good for quick checks but not reliable.